1. Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method therefor, and in particular, a liquid crystal display device including a condenser lens and a manufacturing method therefor.
2. Description of the Related Art
In recent years, in-plane switching (IPS) and vertical alignment (VA) transmissive liquid crystal display devices having a wide viewing angle are widely in use for monitors for various apparatuses and televisions. In addition, liquid crystal display devices are widely used for portable information apparatuses, including portable phones and digital cameras, because the liquid crystal display devices are light-weight. However, further reduction in the thickness and weight are required for display devices for portable information apparatuses, as portable information apparatuses are getting lighter. The thickness of most liquid crystal display devices for portable information apparatuses is reduced during a manufacturing process, by polishing a glass substrate of a liquid crystal panel. Methods of polishing glass substrates generally include chemical polishing using hydrofluoric acid or the like, and mechanical polishing of physically polishing using an abrasive.
In addition, display devices for portable information apparatuses are used in various environments in terms of brightness, including outdoors in clear weather and in dark indoors. For this reason, transflective liquid crystal display devices having a reflective display portion and a transmissive display portion in each pixel have been developed. In the transflective liquid crystal display devices, the transmissive display portions display an image using a backlight, similarly to conventional liquid crystal display panels, and the reflective display portions use reflection of external light for display. However, the backlight is blocked in the reflective display portions, and therefore, there is a problem that the numerical aperture is small in comparison with total transmissive liquid crystal display devices.
The resolution of the liquid crystal display devices for portable information apparatuses has been increasing together with opportunities for one segment reception service for digital terrestrial broadcasting for portable phones and mobile terminals and high-resolution photography. There is a limit to the size of thin film transistors for driving liquid crystal and miniaturization of the width of wires, and hence an area occupied by wires of a substrate (TFT substrate) on which a thin film transistor (TFT) is formed becomes larger as the resolution increases. Wires are formed of such materials as metals which do not transmit light, and therefore block the backlight. Therefore, the area of the aperture through which light transmits becomes smaller as the resolution increases.
As described above, the ratio of light from the backlight which transmits to the front of the display device tends to decrease in display devices for portable information apparatuses. As a method of avoiding this problem, there is a technology for forming condenser lenses on outer surface of the TFT substrate so that light from the backlight is condensed in the light aperture, and thus, light from the backlight which would otherwise be blocked by the wires of the TFT substrate and the reflective portion may be used effectively.
Documents related to the invention of this application include JP2003-337327A, JP2008-304523A, JP2007-25458A, and JP2007-133037 A. JP 2003-337327 A discloses a liquid crystal display device in which a condenser plate with a row of condenser lenses for condensing light from a backlight to apertures in pixel electrodes is stuck to a backlight side of a liquid crystal panel. However, because the condenser plate has the thickness of its base in addition to that of the condenser lenses, forming just lenses on a liquid crystal panel is a superior method from the standpoint of making a display device thinner. JP 2008-304523 A discloses a structure in which condenser lenses are formed directly on a polarization film by off set printing. JP 2007-25458 A discloses a technology of forming condenser lenses directly on an outer surface of a liquid crystal panel, without the intervening polarization film, by photolithography that utilizes a pixel matrix pattern of the liquid crystal panel. Several other technologies of forming lenses directly are known, including mold transcribing methods, ink jet methods, printing methods such as intaglio offset printing, and photolithography-based methods. JP 2007-133037 A discloses a method in which, after condenser lenses are formed directly on an outer surface of a liquid crystal panel, a plurality of protrusions provided at the peaks of the lenses are used as supporters to mount a polarization film.
In the case where condenser lenses are formed on a polarization film stuck to a liquid crystal panel, the polarization film expands and contracts when heated or when the humidity changes, and accordingly causes a change in lens shape or lens pitch, thereby affecting the display performance. Further, optical design puts limitations on the distance from the condenser lenses to light apertures, which makes the thickness of the polarization film an issue in some cases.
In the case where condenser lenses are formed directly on a liquid crystal panel without the intervening polarization film, on the other hand, where to mount a polarization film is an issue. A polarization film is mounted to a liquid crystal panel by sticking a polarization film with an adhesive to an outer surface of the liquid crystal panel. However, in the case where condenser lenses are formed on the outer surface of the liquid crystal panel, the polarization film is stuck onto the condenser lenses and gaps between the condenser lenses are filled with the adhesive of the polarization film. With the adhesive filling the gaps, the condenser lenses which exert a light condensing function by utilizing a difference between a refractive index of air acting as a medium of light and a refractive index of the condenser lenses is significantly lowered in the light condensing function due to a reduced difference in refractive index. The light condensing function may be maintained to some degree by employing a method that increases the refractive index difference between the condenser lenses and the adhesive, or a method in which the gaps between the lenses are filled with a material whose refractive index is high relative to the condenser lenses and then a polarization film is stuck onto the lenses. Still, the lowering of the light condensing function is unavoidable.
In the case where a polarization film is stuck to the backlight side, too, attaching a polarization film to a prism sheet of a backlight which is processed to have a three-dimensional surface in order to guide light lowers the function of the prism sheet for the same reason as described above. Further, because a prism sheet in general is a thin sheet, the expansion and contraction of a polarization film stuck to a prism sheet deforms the prism sheet and may cause a display error.
JP 2007-133037 A discloses a method in which, after condenser lenses are formed directly on an outer surface of a liquid crystal panel, a plurality of protrusions provided at the peaks of the lenses are used as supporters to mount a polarization film. This method may fix a polarization film more firmly than in a structure that uses adhesive layers provided selectively in the perimeter of a liquid crystal panel to stick a polarization film. On the other hand, the method involves a laborious manufacturing process and, due to an air layer held between the liquid crystal panel and the polarization film, creates more interfaces than in a structure where a polarization film is stuck directly to a liquid crystal panel. Accordingly, more light is reflected at interfaces before light from a backlight reaches the front surface of the liquid crystal panel, and the luminance may be lowered as a result.